1. Field of the Invention
The present invention relates to a photoresist pattern forming method, and more particularly to a method of forming a photoresist pattern on a silicide film (of an interconnect metal compound of silicon and a metal element).
2. Description of the Related Art
In the manufacture of transistors and other semiconductor devices, it has been customary to use tungsten silicide (WSi) as conductive material for gate electrodes and wirings. The conventional method for formation of this tungsten silicide film is exemplified by chemical vapor deposition (CVD) and sputtering; with recent progress in miniturization of semiconductor integrated circuits, chemical vapor deposition is increasingly becoming more popular than sputtering as a tungsten silicide film whose resistivity is lower by approximately 20%-40% can be obtained.
Conventionally, in the photolithographic process for the tungsten silicide film formed by CVD, using grays (436 nm in wavelength) and i rays (365 nm in wavelength) of a high-voltage mercury lamp as a light source, exposure takes place over a novolak-based photoresist coated and solidified on the tungsten silicide film to form a photoresist pattern. Recently, however, the design requirement for semiconductor integrated circuits has come down below 35 nm. In an effort to obtain a preciser photoresist pattern to meet such design criterion, exposure using a krypton fluoride (KrF) excimer laser as a light source (248 nm in wavelength) is realized. In this exposure using KrF excimer laser, it has become practical to use a chemical amplification photoresist instead of the novolak-based photoresist, which is too large in light absorption to obtain a good resist pattern.
A chemical amplification photoresist is a positive photoresist to which an acid catalyst reaction is applied. The positive photoresist is generally composed of a base resin, such as polyhydroxystyrene, which is to be insoluble with an alkali developing liquid when the protection radical is in a bonded state at a predetermined portion and to be soluble with the alkali developing liquid when the radical is in a free state, a photoionizer, which emits hydrogen ions upon exposure to light, a small mass of additive for controlling the performance, and an organic solvent for spinner-coating.
When this chemical amplification photoresist coated on the tungsten silicide film and dried to become solid is exposed to irradiation of far ultraviolet rays as of a KrF excimer layer light source, hydrogen ions are generated from the photoionizer to start chemical amplification. When the hydrogen ions are bonded with the base resin in place of the protection radical of the base resin during post exposure baking (PEB), the base resin will be soluble with the alkali developing liquid. In the meantime, the freed protection radical will also reacts with water to generate hydrogen ions again so that the above-mentioned dissolving with the alkali developing liquid will be expedited in chain reaction.
Therefore, if the positive chemical amplification photoresist is developed by the alkali developing liquid, a desired photoresist pattern can be obtained even with insufficient exposure.
The manner in which the chemical amplification photoresist is patterned according to the conventional phtolithographic process will now be described in further detail with reference to FIGS. 3(a) through 3(c) of the accompanying drawings.
Firstly, a silicon oxide film 32 is formed on the surface of a silicon substrate (wafer) 31 as by thermal oxidation. Then a polysilicon film 33 is grown on the silicon oxide film 32 as by CVD. Subsequently, a tungsten silicide (WSi) film 34 is formed by CVD using dichlorosilane (SiH2Cl2) and tungsten hexafluoride (WF6) as raw material (FIG. 3(a)). However, since this tungsten silicide film 34 is very large in internal stress, it is necessary to terminate supplying tungsten hexafluoride, namely, to supply only dichlorosilane at the final stage of such film forming process to intend a reduced stress.
Then, in order to increase adhesion of the photoresist, the semiconductor substrate is exposed to hexamethyldisilazane (HMDS) atmosphere to make the substrate surface hydrophobic. Subsequently, an Si-rich tungsten silicide film 35 is coated with a chemical amplification photoresist 37 (FIG. 3(b)). Then, pattern exposure takes place using KrF excimer laser, for example, as a light source, and finally, only unnecessary portions of the photoresist are removed to form a photoresist pattern 38 (FIG. 3(c)).
However, this conventional photoresist pattern forming method has the following problems:
As is mentioned above, in order to relax internal stresses of the tungsten silicide film 34, at the final stage of formation of the film, supply of tungsten hexafluoride is terminated, namely, only dichlorosilane is continued to be supplied. Even during this final stage, the tungsten silicide film 34 is slightly formed as an Si-rich tungsten silicide film 35 containing silicon excessively (FIG. 3(a)).
This Si-rich tungsten silicide film 35 contains also a high-concentration of chlorine originating from dichlorosilane. Therefore, as is mentioned above, during the surface treatment of the Si-rich tungsten silicide film 35 with hexamethyldisilazane (HMDS) prior to the coating of the chemical amplification resist 37, a chemical reaction occurs between ammonia generated from hexamethyldisilazane and chlorine coming from the uppermost layer of the Si-rich tungsten silicide film 35 to form a solid 36 of ammonium chloride (NH4Cl) as shown in FIG. 4(a).
If the chemical amplification resist 37 is coated to the resulting surface of the Si-rich tungsten silicide film 35 and solidified there (FIG. 4(b)) and then exposure by far ultraviolet rays from a light source as of KrF excimer laser takes place, hydrogen ions serving as a seed to start chemical amplification are generated from the photoionizer in the chemical amplification photoresist and are captured by the solid 36 of ammonium chloride so that chain reaction of release of the protection radical will be obstructed. As a result, the chemical amplification photoresist around the solid 36 of ammonium chloride will be free from so decomposing as to be soluble with alkali. Therefore, during the developing, the photoresist to be removed remains unremoved as pattern defects 39 (FIG. 4(c)); even if the tungsten silicide film 34 is etched in the presence of these pattern defects 39, precise gate electrodes and wirings as designed cannot be achieved, which might result in products defective yet in electrical property.
With the foregoing prior art problems in view, it is an object of the present invention to provide a method of forming a photoresist pattern without causing pattern defects while a metal silicide film formed on a substrate, especially a tungsten silicide film formed by CVD is patterned.
According to a first aspect of the present invention, the above object is attained by a method of forming a photoresist pattern, comprising the steps of: forming a metal silicide film on a substrate; etching a surface layer of the metal silicide; coating the metal silicide film with a photoresist so as to form a photoresist film on the etched surface layer of the metal silicide film; and patterning the photoresist film by photolithography.
Preferably, the etching of the surface layer of the metal silicide film is carried out with a liquid containing hydrogen peroxide in a predetermined concentration before the coating of the photoresist. The substrate is preferably a silicon substrate.
According to a second aspect of the invention, the method further includes the step of making the surface layer of the metal silicide film hydrophobic before the coating of the photoresist and subsequent to the etching of the surface layer of the metal silicide film.
Preferably, the step of making the surface layer of the metal silicide film hydrophobic is carried out by exposing the surface layer of the metal silicide film to hexamethyldisilazane atmosphere diluted with 1-50% of xylene.
As another preferable feature, the photoresist is a chemical amplification photoresist having a base resin, which is to be insoluble with an alkaline solvent when a protection radical is in a bonded state at a predetermined portion and to be soluble with the alkaline solvent when the protection radical is in a free state, and a photoionizer, which emits hydrogen ions upon exposure to light, the base resin being soluble with the alkaline solvent when the protection radical is freed in response to reaction of the hydrogen ions, which are emitted by said photoionizer, and the base resin.
According to a third aspect of the invention, the etching liquid contains at least ammonia, hydrogen peroxide and water.
The etching liquid may be an alternative liquid containing at least sulfuric acid and hydrogen peroxide.
As another alternative, the etching liquid may be a mixed liquid of hydrochloric acid and hydrogen peroxide or its diluted liquid.
As still another alternative, the etching liquid may be a mixed liquid of fluoric acid and hydrogen peroxide.
According to a fourth aspect of the invention, the metal silicide film is a tungsten silicide film.
As an alternative, the metal silicide film may be a titanium silicide film.
Preferably, the tungsten silicide film is formed of at least dichlorosilane (SiH2Cl2) and tungsten hexafluride (WF6) as raw material.